[llvm] [InstCombine] Add transforms for `icmp uPred (trunc x),(truncOrZext(y))` -> `icmp uPred x,y` (PR #71309)

[via llvm-commits]

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@@ -1545,6 +1545,59 @@ Instruction *InstCombinerImpl::foldICmpTruncConstant(ICmpInst &Cmp,
   return nullptr;
 }
 
+/// Fold icmp (trunc X), (trunc Y).
+/// Fold icmp (trunc X), (zext Y).
+Instruction *
+InstCombinerImpl::foldICmpTruncWithTruncOrExt(ICmpInst &Cmp,
+                                              const SimplifyQuery &Q) {
+  if (Cmp.isSigned())
+    return nullptr;
+
+  Value *X, *Y;
+  ICmpInst::Predicate Pred;
+  bool YIsZext = false;
+  // Try to match icmp (trunc X), (trunc Y)
+  if (match(&Cmp, m_ICmp(Pred, m_Trunc(m_Value(X)), m_Trunc(m_Value(Y))))) {
+    if (X->getType() != Y->getType() &&
+        (!Cmp.getOperand(0)->hasOneUse() || !Cmp.getOperand(1)->hasOneUse()))
+      return nullptr;
+    if (!isDesirableIntType(X->getType()->getScalarSizeInBits()) &&
+        isDesirableIntType(Y->getType()->getScalarSizeInBits()))
+      std::swap(X, Y);
+  }
+  // Try to match icmp (trunc X), (zext Y)
+  else if (match(&Cmp, m_c_ICmp(Pred, m_Trunc(m_Value(X)),
+                                m_OneUse(m_ZExt(m_Value(Y))))))
+
+    YIsZext = true;
+  else
+    return nullptr;
+
+  Type *TruncTy = Cmp.getOperand(0)->getType();
+  unsigned TruncBits = TruncTy->getScalarSizeInBits();
+
+  // If this transform will end up changing from desirable types -> undesirable
+  // types skip it.
+  if (isDesirableIntType(TruncBits) &&
+      !isDesirableIntType(X->getType()->getScalarSizeInBits()))
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goldsteinn wrote:

Or you mean it may regress on something like x86 b.c of the `sext` that typically follows to ele width?

https://github.com/llvm/llvm-project/pull/71309